1. Field of the Invention
This invention relates to infusion pump driver devices adapted to pump fluid from disposable cassettes having pumping chambers formed therein from which fluid is displaced by a plunger or piston controlled by the driver device, and more particularly, it pertains to the driving plunger or piston arrangement for such pump driver devices.
2. Description of the Prior Art
In recent years, there has been a considerably increased use of positive displacement infusion pumping devices for delivering fluids intravenously or intra-arterially to patients in hospitals or other patient care locations. These have, to a large extent, replaced the time honored gravity flow control systems primarily due to their much greater accuracy in determining delivery rates and dosages, their relative sophistication in permitting a flexible and controlled feed from multiple liquid sources, and particularly, their ability to control with precision the amounts of dangerous drugs delivered to a patient over a given period of time.
A typical positive displacement fluid infusion pump system will be comprised of a pump driver device and a disposable cassette. The disposable cassette, which is adapted to be used only for a single patient and for one fluid delivery cycle, is typically a small plastic unit having an inlet and outlet adapted to be respectively connected through flexible tubing to the fluid supply container and to the patient receiving the infusion. The cassette will include a pumping chamber with the flow of fluid through the chamber being controlled by a plunger or piston activated in a controlled manner by the driver device. For example, the cassette chamber may have one wall thereof formed by a flexible diaphragm which is reciprocated by the plunger in the driver to cause fluid flow. The pump driver device may include primarily the plunger or piston for controlling the flow of fluid into and out of the pumping chamber in the cassette and it will also include all of the various electronics and control mechanisms to assure that the fluid is delivered to the patient at a pre-set rate, in a predetermined manner, and ony for a particular pre-selected time or total dosage. The pump driver device may also include pressure sensing and other liquid flow monitoring devices as well as valving members for opening and closing various passages in the cassette including the inlet and outlet passages of the pumping chamber.
One form of prior art fluid infusion pumping system is shown in U.S. Pat. No. 4,474,309 to Solomon wherein the cassete includes a pumping chamber which is controlled by a plunger extending from and actuated by the pump driver device through a stepping motor. The plunger, which is moved in a series of incremental movements during each liquid displacement stroke, is driven through a cam which translates the rotary motion of the stepping motor drive shaft to a linear plunger stroke. This cam type of drive inherently results in non-linear movements, and, in order to compensate, the incremental movements at the beginning and end of the plunge stroke, which are significantly smaller than the incremental movements during the major portion of the stroke, are utilized during the refill and initial catch-up phases of the pumping cycle so that relatively linear fluid delivery can be maintained over the major portion of the delivery phase.
U.S. Pat. No. 3,985,133 to Jenkins et al. and U.S. Pat. No. 4,396,385 to Kelly et al. both disclose infusion pump systems wherein the disposable cassette member thereof comprises a pumping chamber provided with a piston at one end and a three-way valve at the other end for selectively connecting the pumping chamber to the liquid supply container or to the liquid delivery tube, i.e., to the patient. The pump driver device includes a valve actuator member for activating the three-way valve in a controlled manner and a piston actuator member for providing a linear reciprocating drive movement to the piston. The piston actuator member is driven by a stepping motor within the pump driver device through a laterally offset connection threadedly secured to the motor drive shaft to translate the rotary movement of the motor drive shaft into linear movement of the actuator. While threaded connections between the drive motor and the infusion pump pistons or plungers have the advantage of uniform, precisely controlled incremental movements, they suffer the disadvantage in that wear between the contacting threaded members can result in a looseness in the drive which can give rise to errors or to a noisy operation which may be detrimental in the normal hospital environment.